Radio receiving system



Dec'. 6, I932. v. c. MACNABB RADIO RECEIVING SYSTEM Filed July 25, 19302 SheetskSheet 1 Icon FIPEQl/i/VC) /N KC.

Dec. 6, 1932.

V. c. MACNABB, 1,890,426

RADIO RECEIVING SYSTEM Filed July 25, 1930 2 Sheets-Sheet 2 Patented5,11 932 I UNITED STATES VERNON O. MACNABB, OF PHILADELPHIA,

PATENT IOFFI'CE' PENNSYLVANIA, 'ASSIGNoB TO ATWA'IEB KENT MANUFACTURINGCOMPANY, OF' PHILADELPHIA, PENNSYLVANIA, A COB- PORATIONv OFPENNSYLVANIA RADIO RECEIVING SYSTEM.

Application tiled July 25,

1 vacuum tube oscillator, the input circuit, re-

trating the characteristics of a system genergenerally as described inFig. 1. Thereis 100' generatively magnetically coupled to the outputcircuit, comprises a loop circuit in which there are serially related aninductance, a variable frequency-determining condenser and anothercondenser, preferably of fixedcapacity, across whose terminals areconnected, through an inductance coupled to the aforesaid inductance,the input electrodes of the oscillator tube.

Further in accordance with my invention, in an oscillator system of thecharacter aforesaid, a resistance is shunted across the seriallyconnected fixed capacity and second inductance.

Further in accordance withmy invention,

in a heterodyne or superheterodyne receiving system a vacuum tubeoscillator of the char acter referred to is. utilized for producingoscillations superimposed upon the signalrepresenting oscillations; andmore particularly, the locally generated oscillations are impressed uponor in advance of the inputthe oscillations is offset because in myoscil-.

lator system the amplitude of the locally produced oscillationsdecreases at uniform rate with increase of their frequency.

My'invention resides in a system of the character hereinafter describedand claimed. For'an understanding ofmy invention'and I for anillustration of some of the various embodiments thereof, reference is tobe had to the accompanying drawings, in which:

Fig. 1 is a diagrammatic illustration of the commonly used oscillatorsystem;

Fig. 2 comprises curves or graphs illus- 1930. Serial No. 470,557.

ally illustrated by Fig. 1 and of my improved system;

Fig. 3 is a diagram of an embodying my invention;

Fig. 4 is a diagram of a portion of a superheterodyne radio receivingsystem embodying my invention.

Referring to Fig. 1, O is a thermionic vacuum tube having control grid9, anode or plate a, and cathode or filament f maintained at suitabletemperature by current from the source 1. In thev grid or input circuitis an I inductance L1, shunted by. the variable conoscillator system 7denser C, forming a tunable or resonant loop whose terminals areconnected to the. grid and cathode. In the plate ,circuit is the usualsource 2 of plate circuit current and an inductanceQL2. coupled to L1.This system comprises magnetically coupled input and outputs stems,causing regeneration to the oint"o production of oscillations whoserequenc substantially entirely, dependent u on the capacity of the:variable condenser substantial extent dependent upon variations in platecircuit voltage, voltage of and current from the source 1,characteristics of the tube 0, and. other factors. Coupled to theinductance L1 is an inductance or coil L3 across whose terminals appearsa voltage which may be impressed upon theinput circuit of a heterodyneor superheterodyne system, and which increases rapidly as the condenseris adjusted to smaller and smaller capacities to produce oscillations ofhigher and higherfrequencies. 1 a

1 he curve A, Fig. 2, in ageneral way represents the voltage-frequencycharacteristic of such an oscillator. The voltage impressed is tosubstantial extent, but'not The frequency of the generated oscillationsis to upon the detector in such case rises rapidly with increasingfrequency, andthis'is unde sir-able, particularly in superheterodynereception.

In accordance with my invention, the -vacuum tube-oscillator is of thecharacter illustrated in Fig. 3, in which the inductances or coils, L1,L2 and L3 are related to each other 'orinductan'ce L4 coupled to thecoil L1. coils LI and L4 may be considered as a single windlng or coil,wound throughout .in the the significant variation, however, in that inlieu of connecting the input electrodes f and g to the terminals of theresonant loop L1 C as in Fig. 1, there is included in the resonant loopan additional condenser C1, preferably of fixed capacity, across whoseterminalsthe input electrodes are connected through a coil samedirection and tapped at the point 3. So arranged the coupling of coil L4with coil L1 is in the, proper sense for the preferred purposes of myinvention. It will be-understood that any equivalent arrangement of acoil coupled to L1 and in series with the condenser Cl across the inputelectrodes of the tube 0, is contemplated.

If the coil L4 were omitted or not utilized, as by connecting the lead 6directly to the point 3 or lower terminal of the coil L1, the condenserC1 of the resonant loop would alone be connected across the inputterminals of the tube 0. That arrangement is disclosed in Millerapplication, Serial No. 470,016, filed July 23, 1930. My system, Fig. 3,preferably embodies. the relations, proportions and characteristics ofthe system, disclosed in said Miller application. My invention is animprovement over the system disclosed in that application, by inclusionin series with the condenser (11 of a coil, such as L4, suitably coupledto another coil of the system, and across whose terminals exists adifference of potential which coacts with the difference of potentialacross the terminals of the condenser C1, preferably in such way thattheoscillatory current or amplitude of the oscillations produced so de-.creases with increase in their frequency that the voltage across theterminals of a secondvoltage across the terminals of the secondary L3actually decreases with further increase of frequency, as indicated bythe portion B2 'of the curve B.

By utilizing the coil L4 or equivalent, the relation of voltage acrossthe terminals of the secondary L3 to the frequency of the oscillationsgenerated is practically linear throughout the entire frequency range asindicated by the characteristic curve F, Fig. 2.

The

My system will produce oscillations when M Gm Gm w? M M1 is equal to orgreater than unity; where is the mutual inductance of the couplingbetween coils L1 and L2, Gm is the mutual conductance of the tube 0-, Clis the capacity of the condenser Cl, M1 is the mutual inductance of thecoupling between the coils L1 and L4, to is 2 7r times the frequency,and R is the high or radio frequency resistance effectively in series inthe resonant loop L1, C, C1. The resistance R is of maximum magnitude atthe highest frequency of the generatedvoscillations and decreases withdecreasing frequency.

The voltage impressed upon the grid and cathode of the tube 0 may beeither the sum of or the-difference between the voltage across theterminals of the condenser Cl and the voltage across the terminals ofthe coil L4, depending upon the sense of coupling of the coil L4 to thecoil Ll. With one sense of coupling the amplitude of the oscillationsproduced increases with increase of frequency, and with the oppositesense of coupling decreases with increase of frequency.

' As indicated by the characteristic B of 2, in the case where the coilL4 is omitted,

throughout the straight portion Bl, the amplitude of the oscillationsdecreases with increase of frequency, and throughout the portion B2, forthe higher frequencies of the range, the amplitude of the oscillationsdecreases at higher rate with increasing fre-,

quency.

For the range of frequencies corresponding with the portion B1, theextra winding or coil L4, with the preferred sense of coupling abovedescribed, is neither so effective nor so necessary as for that portionof the frequency range covered by the portion B2 of the characteristicof the circuit without the coil L4.

Throughout the higher frequency portion of the range, the voltage acrossthe terminals of the coil L4 becomes-more and more effective, operatingcumulatively with the voltage across the terminals of the condenser C1,and

so affecting the decrease in amplitude of the oscillations; for thatportion of the frequency range, that the voltage across the terminals ofthe'secondary L3, also coupled to the coil L1, does, not at such greatrate decrease with mcrease of frequency, and the characteristic F forthe entire frequency range becomes practically linear, with but small orinsubstantial change of voltage across the secondary L3 with change offrequency.

When thesense of coupling of the coil L4 to the coil L1 is opposite tothe preferred sense above referred to, the voltage across the terminalsof the coil L3 would decrease rapidly with increase of frequency,particularly throughout the higher frequency portion of 1 J thecharacterdescribed is of general applicautilized a resistance R1 bridgedacross the series combination Cl L4, in lieu of across the terminalsof-the condenser Cl only when the coil L4 is absent.

My novel oscillation producing system of tion. It is especiallysuitable, however, for a system of the superheterodyne type forbroadcast reception of speech and music. In such case the frequency ofthe oscillations produced may range from 680 to 1630 kilocyclesfor'producing a beat or intermediate frequency of 130 kilocycles byinteraction with the signal oscillations of broadcast frequency rangingfrom 550-to 1500 kilocycles.

Fig. 4 represents a portion of asuperheter ,odyne receiving systemembodying a local oscillator in accordance with my invention and of thecharacter illustrated by Fig. 3,

In Fig. 4, D represents an antenna or other suitable absorptionstructure. In this case between the antenna D and earth E or ground,

or equivalent counter capacity, or the metal chassis of a receiving set,is included the primary p to different taps on which connects theantenna switch I) to suit the receiving system to antennas of differentcharacteristics or lengths. Coupled to the primary p is the secondary s,in circuit with which is the pri- 'mary 121. In shunt to s and 721 isthe tuning condenser C2 for tuning the circuit to the frequency of thesignal-representing oscillations.

Coupled to the primary 191 is the secondary s1,

in series with which is an inductance .92,- both shunted or bridged bythe condenser C3 vari-. able to tune this cascaded circuit to thefrequency of the signal representing oscillations.

In shunt to the inductance 82 is a condenser K.

V is the first detector, of a superheterodyne system, having a controlgrid 9, cathode c, maintained at suitable temperature by the electricheater h, and the plate or anode a, and shield or screen grid d. Thecontrol grid is connected to a pointbetween the induct? ances s1 and 82,while the cathode is connected through the coil or secondary L3 and gridbiasing resistance R2, shunted by condenser K1, ,to ground E, or otherterminal of the loop tuned by the condenser C3.

' To the output circuit of the detector is coupled the input circuit ofan intermedlate frequency amplifier tube, with which may be cascaded oneor more similar intermediate frequency amplifier tubes, the platecircuit of the last of which is coupled to the input of the seconddetector tube, whose output cir-- cuit is operatively related to asignal translating instrument, commonly a loud speaker,

either directly or generally through one or more stages of audiofrequency amplification.

The intermediate frequency amplifiers, second -detector, etc., are notillustrated, since that arrangement is well known.

The local oscillator O 'in thisin'stance is provided with a cathode 0,heated by the ele'c its'cathode a'filament f as in Fig. 3. The

parts of the oscillator are generally the same with the action andcharacteristics of a system such as illustrated by Fig. 3. The platecircuit is coupled by coil L2 to the coil Ll of the input circuit underconditions producing oscillations. pu-t circuit is coupled with theinductance L1, to impress upon the detector V a voltage of radiofrequency difiering to fixed extent from the signal frequencies to whichthe condensers C2 and C3 tune their'respective circuits. As indicated bythe broken line U, the rotors or adjustable elements of the threecondensers C, C2 and C3 are mechanically coupled in unison, so that asthe cascaded circuits are I tuned to the signal frequency, the condenserC is varied as to its capacityin such amount as to cause the frequencyof the oscillations produced by generator 0 always to differ to fixedextent from the signal frequency, there'- a by yielding a constant beator intermediate frequency.

In'the example illustrated, for the power for heating the cathodes andsupplying current .to the plate circuits of the various tubes of theset, there is provided a transformer T whoseprimary is connected acrossthe commercial alternating current lighting or power lines 4 and 5. Thesecondary S has its terminals connected to the anodes a of the rectifiertube Vlwhose filament or cathode f receives its current from thesecondary S1. The rectified current is passed through a filter systemcomprising a suitable arrangement of inductances andcapacities,'yielding at the terminals 6 and 7 a rectified filteredcurrent the tubes in'the system. In the plate circuit of the oscillatorO, in series with the'coil L2,

is a resistance R3, shunted by condenser K2,

'of suitable magnitude, say 10,000 ohms, -which keeps the plate currentof the tube 0 from rising to h gh magnitude, or regulates the platecurrent. .The resistance R3 is of such magnitude that the effectivevoltage imis of the order of, say '100 volts, the tube 0 being of thetype now commonly known as UY 227 or C 327-.

pressed upon the plate circuit of the tube 0 The secondary S2 of thetransformer T "supplies current for the heaters h, of the several tubes,for maintaining their cathodes at suitable temperature. In lieu ofcathodes 0,

filaments such as f, Fig. 3, may be utilized,

- in which case they receive their current from the secondary S2, or asecondary similar or in addition thereto.

By Way of illustrative example of proportions suitable for my pu oses,the coils L1, L2, L3 and L4 may have lnductances of 190, 60, 3.8 and 9microhenries, respectively. The mutual inductance M, between the coilsL1 and L2, may be 5 5 microhenries; the mutual inductance between coilsL1 and L3, 14.3 microhenries; and the mutual inductance M1, between thecoils L1 and L4, 15 microhenries. The capacity of the condenser C may bevaried from to 400 micro-microfarads; the capacity of condenser C1 maybe 1200 micromicrofarads; and resistance R1 of the order of 50,000 ohms.The plate voltage of the tube 0 may be from about 100 to about 110volts. The capacities of the tuning condense ers C2 and C3 may bevariable from 50 to 400 micro-microfarads for tuning to signalfrequencies ranging from 550 to 1500 kilo? cycles. The three variablecondensers are adjustable in unison to effect a constant beat orintermediate frequency, which is possible because the frequency of theoscillations produced by the oscillator O is governed subcharacteristics hereinbefore described,and in particular-that thefrequency of the locally generated oscillations shall be dependentsubstantially entirely upon or strictly related to the capacity of thevariable condenser C; It is particularly. desirable that the oscillatorhave this characteristic when utilized in a 'superheterodyne systemprovided with highly selective systems, one in advance of the firstdetector and tunable to the desiredsignal frequencies, and the otherfollowing the first detector and highly selective to the beat frequency.For this purpose it is of importance and highly desirable, particularivin a uni-control system, such as'described, that the frequency of theoscillator always difi'ers to, substantially strictly the same ex-" aninductance. in series with serially con nected capacities, one of saidcapacities being variable and having a maximum less than another of saidcapacities, means for coupling 0 the output system of said tube to saidinductance, and connections to the input elecl pling the output systemof said tube to said inductance, and connections to the input electrodesof said tube from the terminals of said other capacity through aninductance coupled to said first named inductance, the volt-I agesacross said other capacity and said last named inductance operatingcumulatively to cause the amplitude of the generated oscillations todecrease substantially proportionately to increase of their frequency.

3. In a heterodyne receiver, a detector,

means for impressing signal-representing oscillations on said detector,a local generator of oscillations comprising a vacuum tube, a

loop in its input system comprisingan inductance in series with seriallyrelated capacities, one of which is variable and whose maximum capacityis relativelysmaller than ,another of saidcapacities, means for cou-'pling the output system of said tube to said inductance, connectionsfrom the input electrodes of said tubes to the terminals of said othercapacity through an inductance coupled to. said first named inductance,and an inductance coupled to said first named inductance for impressingthe locally generated oscillations upon said detector.

4. In a heterodyne receiver, a detector, a" selector system in advanceof said detector tunable to desired signal fre uencies by at least onevariable capacity, a coal oscillator comprising a vacuum tube a loop inthe input system of said tube comprisin an 'inductance in series withserially re ated capacities, one of which is variable and whose maximumcapacity is relatively smaller than another of said capacities, all inseries with each other, a connection from the terminals of said othercapacityto the input electrodes of said tube through an inductancecoupled to said first named inductance, means for coupling the outputsystem of said tube to said inductance, an inductance coupled. to saidfirstnamed inductance for impressing 21. voltage upon said detector, andmeans for adjusting said variable capacities in unison to cause thefrequencies'of the locally generated oscillations to difier to fixedextent from said signal frequencies.

. VERNON O. MACNABB.

